1. Field of the Invention
The present invention relates to a signal transmission/reception method for use in a communication system based on an orthogonal frequency division multiplexing (OFDM) scheme, and more particularly to a sequence generation method for allowing a reception end to effectively detect a sequence used for a specific channel of mobile communication system, and a signal transmitting/receiving method using this sequence generation method.
2. Discussion of the Related Art
The OFDM, OFDMA, and SC-FDMA schemes for use in the present invention will hereinafter be described in detail.
In recent times, as the demand of high-speed data transmission rapidly increases, the OFDM scheme is more advantageous to this high-speed transmission, so that the OFDM scheme is used as a transmission scheme for use in a variety of high-speed communication systems.
The OFDM (Orthogonal Frequency Division Multiplexing) scheme will hereinafter be described.
OFDM Scheme
According to the basic principles of the OFDM scheme, the OFDM scheme divides a high-rate data stream into many slow-rate data streams, and simultaneously transmits the slow-rate data streams via multiple carriers. Each of the carriers is called a sub-carrier.
In the OFDM scheme, the orthogonality exists between multiple carriers. Accordingly, although frequency components of the carrier are overlapped with each other, the overlapped frequency components can be detected by a reception end.
More specifically, a high-rate data stream is converted to a parallel low-rate data stream by a serial to parallel (SP) converter. The individual sub-carriers are multiplied by the above parallel data streams, the individual data streams are added to the multiplied result, and the added result is transmitted to the reception end.
On the other hand, the OFDMA scheme is a multiple access method for allowing the OFDM system to allocate the sub-carriers in a total band to each of a plurality of users according to a transmission rate required by each user.
The conventional SC-FDMA (Single Carrier-FDMA) scheme will hereinafter be described. This SC-FDMA scheme is also called a DFS-S-OFDM scheme.
SC-FDMA Scheme
The SC-FDMA scheme will hereinafter be described in detail. The SC-FDMA scheme mainly applied to an uplink performs the spreading based on the DFT matrix in a frequency domain before generating the OFDM signal, modulates the spreading result according to the conventional OFDM scheme, and transmits the modulated result.
Some variables are defined to explain the SC-FDMA scheme. “N” is indicative of the number of sub-carriers transmitting the OFDM signal. “Nb” is indicative of the number of sub-carriers for a predetermined user. “F” is indicative of a Discrete Fourier Transform (DFT) matrix, “s” is indicative of a data symbol vector, “x” is indicative of a data dispersion vector in the frequency domain, and “y” is indicative of an OFDM symbol vector transmitted in the time domain.
Before the SC-FDMA scheme transmits the data symbol (s), the data symbol (s) is dispersed, as represented by the following equation 1:x=FNb×Nbs  [Equation 1]
In Equation 1, FNb×Nb is indicative of a Nb-sized DFT matrix to disperse the data symbol (s).
The sub-carrier mapping process is performed on the dispersed vector (x) according to a predetermined sub-carrier allocation technique. The mapping resultant signal is converted into a time-domain signal by the IDFT module, so that a desired signal to be transmitted to the reception end is acquired. In this case, the transmission signal converted into time-domain signal by to the transmission end can be represented by the following equation 2:y=FN×N−1x  [Equation 2]
In Equation 2, FN×N−1 is indicative of the N-sized IDFT matrix for converting a frequency-domain signal into a time-domain signal.
Then, a cyclic prefix is inserted into the signal (y) created by the above-mentioned method, so that the resultant signal is transmitted. This method capable of generating the transmission signal and transmitting the same to the reception end is called an SC-FDMA method. The size of the DFT matrix can be controlled in various ways to implement a specific purpose.
The above-mentioned concepts have been disclosed on the basis of the DFT or IDFT operation. For the convenience of description, the following description will be disclosed without discriminating between the DFT (Discrete Fourier Transform) scheme and the FFT (Fast Fourier Transform) scheme.
If the number of input values of the DFT operation is represented by the modular exponentiation of 2, it is well known to those skilled in the art that the FFT operation can be replaced with the DFT operation. In the following description, the FFT operation may also be considered to be the DFT operation or other equivalent operation without any change.
Typically, the OFDM system forms a single frame using a plurality of OFDM symbols, so that it transmits the single frame composed of several OFDM symbols in frame units. The OFDM system firstly transmits the preamble at intervals of several frames or each frame. In this case, the number of OFDM symbols of the preamble is different according to the system types.
For example, the IEEE 802.16 system based on the OFDMA scheme firstly transmits the preamble composed of a single OFDM symbol at intervals of each downlink frame. The preamble is applied to a communication terminal, so that the communication terminal can be synchronized with the communication system, can search for a necessary cell, and can perform channel estimation.
FIG. 1 shows a downlink sub-frame structure of the IEEE 802.16 system. As shown in FIG. 1, the preamble composed of the single OFDM symbol is located ahead of each frame, so that it is transmitted earlier than each frame. The preamble is also used to search for the cell, perform the channel estimation, and be synchronized in time and frequency.
FIG. 2 shows the set of the sub-carriers which transmit the preamble from the 0-th sector in the IEEE 802.16 system. Some parts of both sides of a given bandwidth are used as the guard band. If the number of sectors is 3, each sector inserts the sequence at intervals of 3 sub-carriers, and “0” is inserted into the remaining sub-carriers, so that the resultant sub-carriers are transmitted to a destination.
The conventional sequence for use in the preamble will hereinafter be described. The sequence for use in the preamble is shown in the following table 1.
TABLE 1IDSequenceIndexcellSector (hexadecimal)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B22201C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E————
The sequence is defined by the sector number and the IDcell parameter value. Each defined sequence is converted into a binary signal in ascending numerical order, and the binary signal is mapped to the sub-carrier by the BPSK modulation.
In other words, the hexadecimal progression is converted into a binary progression (Wk), the binary progression (Wk) is mapped in the range from the MSB (Most Significant Bit) to the LSB (Least Significant Bit). Namely, the value of 0 is mapped to another value of +1, and the value of 1 is mapped to another value of −1. For example, the “Wk” value of the hexadecimal value “C12” at the 0-th segment having the index of 0 is “110000010010 . . . ”. The converted binary code value is −1, −1, +1, +1, +1, +1, +1, −1, +1, +1, −1, +1 . . . .
The sequence according to the conventional art maintains the correlation characteristics among various sequence types capable of being composed of binary codes. The sequence according to the conventional art can maintain a low-level PAPR (Peak-to-Average Power Ratio) when data is converted into another data of a time domain, and is found by the computer simulation. If the system structure is changed to another, or the sequence is applied to another system, the conventional art must search for a new sequence.
Recently, there is proposed a new sequence for use in the 3GPP LTE (3rd Generation Partnership Project Long Term Evolution hereinafter “LTE”) technology, and a detailed description thereof will hereinafter be described.
A variety of sequences have been proposed for the LTE system. The sequences for use in the LTE system will hereinafter be described.
In order to allow the terminal to communicate with the Node-B (i.e., base station), the terminal must be synchronized with the Node-B over a synchronous channel (SCH), and must search for the cell.
The above-mentioned operation, in which the terminal is synchronized with the Node-B and an ID of a cell including the terminal is acquired, is called a cell search process. Generally, the cell search is classified into an initial cell search and a neighbor cell search. The initial cell search process is executed when the terminal is initially powered on. The neighbor cell search is executed when a connection-mode or idle-mode terminal searches for a neighbor Node-B.
The SCH (Synchronous Channel) may have a hierarchical structure. For example, the SCH may use a primary SCH (P-SCH) and a secondary SCH (S-SCH).
The P-SCH and the S-SCH may be contained in a radio frame by a variety of methods.
FIGS. 3 and 4 show a variety of methods capable of involving the P-SCH and S-SCH in the radio frame. Under a variety of situations, the LTE system may configure the SCH according to the structure of FIG. 3 or 4.
In FIG. 3, the P-SCH is contained in the last OFDM symbol of a first sub-frame, and the S-SCH is contained in the last OFDM symbol of a second sub-frame (in FIG. 3, duration of a sub-frame is supposed to have 0.5 ms. But the length of the sub-frame can be differently configured according to the system).
In FIG. 4, the P-SCH is contained in the last OFDM symbol of a first sub-frame, and the S-SCH is contained in a second OFDM symbol from the last OFDM symbol of the first sub-frame (in FIG. 4, also, duration of a sub-frame is supposed to have 0.5 ms).
The LTE system can acquire the time/frequency synchronization over the P-SCH. Also, the S-SCH may include a cell group ID, frame synchronous information, and antenna configuration information, etc.
The P-SCH configuration method proposed by the conventional 3GPP LTE system will hereinafter be described.
The P-SCH is transmitted over the band of 1.08 MHz on the basis of a carrier frequency, and corresponds to 72 sub-carriers. In this case, the interval among the individual sub-carriers is 15 kHz, because the LTE system defines 12 sub-carriers as a single resource block (RB). In this case, the 72 sub-carriers are equal to 6 RBs.
The P-SCH is widely used in a communication system (e.g., an OFDM or SC-FDMA system) capable of employing several orthogonal sub-carriers, so that it must satisfy the following first to fifth conditions.
According to the first condition, in order to allow a reception end to detect a superior performance, the above-mentioned P-SCH must have superior auto-correlation and cross-correlation characteristics in a time domain associated with constituent sequences of the P-SCH.
According to the second condition, the above-mentioned P-SCH must allow a low complexity associated with the synchronization detection.
According to the third condition, it is “preferable” that the above-mentioned P-SCH may have the Nx repetition structure to implement a superior frequency offset estimation performance.
According to the fourth condition, the P-SCH having a low PAPR (Peak-to-Average Power Ratio) or a low CM is preferable.
According to the fifth condition, provided that the P-SCH is used as a channel estimation channel, the frequency response of the P-SCH may have a constant value. In other words, from the viewpoint of the channel estimation, it is well known in the art that a flat response in a frequency domain has the best channel estimation performance.
Although a variety of sequences have been proposed by the conventional art, the conventional art cannot sufficiently satisfy the above-mentioned conditions.